Calculating Percentage by Mass Worksheet (teacher made) - Free Printable

Problem Set Solutions

#### Mass Percent Calculations

1. Calculate the mass percent of a NaCl solution prepared by mixing 47.0 g NaCl with 125.0 mL of pure H₂O.

- Step 1: Determine the total mass of the solution.
- Mass of NaCl = 47.0 g
- Volume of water = 125.0 mL
- Density of water = 1.00 g/mL (assumed)
- Mass of water = Volume × Density = 125.0 mL × 1.00 g/mL = 125.0 g
- Total mass of the solution = Mass of NaCl + Mass of water = 47.0 g + 125.0 g = 172.0 g

- Step 2: Calculate the mass percent of NaCl.
\[
\text{Mass percent of NaCl} = \left( \frac{\text{Mass of NaCl}}{\text{Total mass of solution}} \right) \times 100\%
\]
\[
\text{Mass percent of NaCl} = \left( \frac{47.0 \, \text{g}}{172.0 \, \text{g}} \right) \times 100\% \approx 27.33\%
\]

- Final Answer:
\[
\boxed{27.33\%}
\]

2. What is the mass percent of a sodium fluoride solution prepared by dissolving 0.48 moles of sodium fluoride into 63.5 grams of water?

- Step 1: Calculate the mass of sodium fluoride (NaF).
- Molar mass of NaF = 22.99 g/mol (Na) + 19.00 g/mol (F) = 41.99 g/mol
- Mass of NaF = Moles × Molar mass = 0.48 mol × 41.99 g/mol ≈ 20.07 g

- Step 2: Determine the total mass of the solution.
- Mass of NaF = 20.07 g
- Mass of water = 63.5 g
- Total mass of the solution = Mass of NaF + Mass of water = 20.07 g + 63.5 g = 83.57 g

- Step 3: Calculate the mass percent of NaF.
\[
\text{Mass percent of NaF} = \left( \frac{\text{Mass of NaF}}{\text{Total mass of solution}} \right) \times 100\%
\]
\[
\text{Mass percent of NaF} = \left( \frac{20.07 \, \text{g}}{83.57 \, \text{g}} \right) \times 100\% \approx 24.02\%
\]

- Final Answer:
\[
\boxed{24.02\%}
\]

3. A dioxin-contaminated water source contains 0.085% dioxin by mass. How much dioxin is present in 2.5 L of this water? (Assume that the density of the solution is 1.01 g/mL).

- Step 1: Calculate the mass of the solution.
- Volume of solution = 2.5 L = 2500 mL
- Density of solution = 1.01 g/mL
- Mass of solution = Volume × Density = 2500 mL × 1.01 g/mL = 2525 g

- Step 2: Calculate the mass of dioxin in the solution.
- Mass percent of dioxin = 0.085%
- Mass of dioxin = (Mass percent of dioxin / 100) × Total mass of solution
\[
\text{Mass of dioxin} = \left( \frac{0.085}{100} \right) \times 2525 \, \text{g} = 0.00085 \times 2525 \, \text{g} \approx 2.15 \, \text{g}
\]

- Final Answer:
\[
\boxed{2.15 \, \text{g}}
\]

#### Molarity Calculations

1. What is the molarity of a solution prepared by dissolving 10.7 g NaI in 0.250 L?

- Step 1: Calculate the moles of NaI.
- Molar mass of NaI = 22.99 g/mol (Na) + 126.90 g/mol (I) = 149.89 g/mol
- Moles of NaI = Mass / Molar mass = 10.7 g / 149.89 g/mol ≈ 0.0714 mol

- Step 2: Calculate the molarity of the solution.
- Molarity (M) = Moles of solute / Volume of solution (in liters)
\[
\text{Molarity} = \frac{0.0714 \, \text{mol}}{0.250 \, \text{L}} \approx 0.286 \, \text{M}
\]

- Final Answer:
\[
\boxed{0.286 \, \text{M}}
\]

2. How many moles of KOH are contained in 750. mL of 5.00 M KOH solution?

- Step 1: Convert the volume to liters.
- Volume = 750. mL = 0.750 L

- Step 2: Calculate the moles of KOH.
- Molarity (M) = Moles of solute / Volume of solution
- Moles of KOH = Molarity × Volume
\[
\text{Moles of KOH} = 5.00 \, \text{M} \times 0.750 \, \text{L} = 3.75 \, \text{mol}
\]

- Final Answer:
\[
\boxed{3.75 \, \text{mol}}
\]

3. How many grams of KCl are needed to make 50.0 mL of 2.45 M KCl?

- Step 1: Convert the volume to liters.
- Volume = 50.0 mL = 0.0500 L

- Step 2: Calculate the moles of KCl.
- Molarity (M) = Moles of solute / Volume of solution
- Moles of KCl = Molarity × Volume
\[
\text{Moles of KCl} = 2.45 \, \text{M} \times 0.0500 \, \text{L} = 0.1225 \, \text{mol}
\]

- Step 3: Calculate the mass of KCl.
- Molar mass of KCl = 39.10 g/mol (K) + 35.45 g/mol (Cl) = 74.55 g/mol
- Mass of KCl = Moles × Molar mass
\[
\text{Mass of KCl} = 0.1225 \, \text{mol} \times 74.55 \, \text{g/mol} \approx 9.13 \, \text{g}
\]

- Final Answer:
\[
\boxed{9.13 \, \text{g}}
\]

Final Answers:

1. Mass Percent Calculations:
- 1. \(\boxed{27.33\%}\)
- 2. \(\boxed{24.02\%}\)
- 3. \(\boxed{2.15 \, \text{g}}\)

2. Molarity Calculations:
- 1. \(\boxed{0.286 \, \text{M}}\)
- 2. \(\boxed{3.75 \, \text{mol}}\)
- 3. \(\boxed{9.13 \, \text{g}}\)